Found this good 1995 YA thriller in a 2nd hand store. A very good buy it was too. At a posh boarding school a rich boy and his townie girlfriend break the rules. A nerd blackmails them because of it. Nerd turns out to be a nut as well as a blackmailer. Rich boy, Jack, goes really nuts to put a stop to nerd's nefarious ways. Townie girlfriend, Jill, is caught up in the riptide.
Everyone is an amoral psycho. The ending is grim and nasty. This is like Christopher Pike without the smug.